1. Field of the Invention
The present invention relates to a water proof optical fiber cable for optical communication.
2. Description of the Related Art
A waterproof communication cable which comprises stranded element wires each comprising a copper conductor coated with a plastic coating such as polyethylene- and polypropylene, a waterproof compound filled in spaces in the cable and an outer sheath is used as a maintenance-free communication cable. As the waterproof compound, a compound comprising a polybutene base oil and low molecular weight polyethylene, a compound comprising a petrolatum base oil and low molecular weight polyethylene and the like are known. As disclosed, for example, in Japanese Patent Kokai Publication No. 202015/1982, the waterproof compound is heated and molten and poured in the spaces in the cable. since the conventional waterproof compound is filled in the cable space in the molten state, in case of an optical fiber cable, volume shrinkage of the waterproof compound during cooling generates short period small bending, namely microbending on each optical fiber and increases light transmission loss.
As a waterproof compound for an optical fiber cable Japanese Patent Kokai Publication No. 126706/1986 discloses a waterproof compound for an optical fiber cable comprising a synthetic oil and optionally a mineral oil, and Japanese Patent Kokai Publication No. 8777/1983 discloses a waterproof compound for an optical fiber cable comprising a mineral oil and hydrophobic silica.
In addition to the already described problem encountered in the production of the waterproof optical fiber cable, the conventional waterproof optical fiber cables have following three problems:
(A) Increase of transmission loss at low temperatures.
(B) Deterioration of a coating material of the optical fiber,
(C) Bleeding of the waterproof compound from cable ends.
A cause for the problem (A), namely the increase of transmission loss at low temperature is that the waterproof compound solidifies and shrinks at low temperature and tightens the optical fiber to generate the microbending on the optical fiber.
A cause for the problem (B), namely the deterioration of the coating material of the optical fiber, is that an oil component which is the main component of the waterproof compound tends to penetrate in the coating material of the optical fiber and swells or deteriorates the coating material, since the oil component is a hydrocarbon base oil. For example, it is known from Japanese Patent Kokai No. 100036/1988 that an oil in the waterproof compound penetrates in the coating material of the optical fiber to generate stress therein, whereby the microbending is generated on the optical fiber. To solve such problem, it is proposed to reduce influence of the oil to the coating material by decreasing amounts of aromatic components in the oil and polarity of the oil as much as possible (see, for example, Japanese Patent Kokai Publication No. 213813/1986). Since water absorbance of the coating material of the optical fiber is made low recently and polarity of the coating material is made smaller than ever, it is difficult to prevent swelling of the coating material by suppressing the polarity of the oil component.
The bleeding of the waterproof compound from the cable ends (C) means flowing out of the waterproof compound as such or the oil component in the waterproof compound from the cable ends and decreases workability or makes it impossible to connect the optical fibers.
Hitherto, no waterproof compound can solve all the above problems (A), (B) and (C).